This invention relates to current leads for attachment to a cryogenic apparatus such as a superconducting magnet or instrument positioned in a cryostat. The cryogenic materials used to cool the superconducting magnet or instrument may be liquid helium, liquid hydrogen, liquid neon, or other coolants, these materials being well known in the cryogenic art. A cryogenic apparatus, such as a superconducting magnet or low temperature sensor, typically requires several current-carrying leads to supply power to the circuit or to read information into or out of the circuit. These leads introduce heat into the cryostat by thermal conduction and, if the leads are not superconducting, by Joule heating.
It is desirable to minimize the heat input into the cryostat in order to reduce the amount of cryogenic fluid boiled off in an open system or to reduce the power consumption in a cryostat cooled by a closed-cycle refrigerator.
Future space missions will include sensors and instruments operating at cryogenic temperatures, that is temperatures less than 77.degree. K. In addition, long-term inorbit storage of liquid oxygen and liquid hydrogen will be required for propulsion by missions to the moon or to Mars or by orbit transit vehicles in Earth orbit. The heat leak into the stored cryogens, along with the heat leak due to or carried by current leads to the cold sensors and instruments can greatly increase the weight and power overhead associated with these cryogenic systems or reduce their lifetimes. Because both weight and power requirements are cost drivers for these missions, and because long lifetimes are required, cryogenic current leads with low heat input are an important means by which both weight and power requirements can be reduced and hence, by which the lifetime of the mission can be increased.
Many terrestrial superconducting devices will profit from a reduction of the heat loss associated with current leads. For example, it has been estimated that, for a five thousand MWh superconducting magnet energy storage system, the use of high temperature superconducting material for the leads has the potential to reduce the parasitic refrigeration losses of the system by more than half. Even for small laboratory superconducting magnets, the loss of helium in a normal operation of the magnets is often a concern. Frequently, it is not cost effective to recover all the helium that evaporates in the system so that a reduction in the amount of the helium evaporated due to introduction of heat into the cryostat is valuable.